The present invention relates to a method and device for digital signal reproduction, and particularly to a signal reproduction method and device for a digital video signal reproduction apparatus (will be termed simply "digital VTR" (Video Tape Recorder) hereinafter) having a slow-motion playback performance based on a memory.
A conventional digital VTR is disclosed in U.S. Pat. No. 4,392,162 for example. The above-mentioned digital VTR operates to record a video signal on a magnetic tape so that each field of signal is divided into a plurality of tracks. In the slow-motion playback operation for a magnetic tape on which the video signal is recorded, a memory with a capacity of one field or one frame is used to record the video signal which is retrieved from the magnetic tape at a lower tape speed, and then the stored video signal is read out of the memory. Storing of the reproduced video signal into the memory in the slow-motion playback mode is carried out in the following manner. A video signal is made up of segments of a unit time length, such as the horizontal period (will be termed "blocks" hereinafter), and each block has an appendage of an index (will be termed simply "ID" hereinafter) signal which indicates the field number and frame number to which the block belongs. The reproduced video signal is rendered the block-wise error detection and error correction process or the like, and blocks which are rid of error are stored in address locations corresponding to the ID signals in the memory. Based on the storage of reproduced video signals without error in the memory, a slow-motion picture produced by the video signals read out of the memory is flawless or errorless.
Next, the quarter-speed slow-motion playback operation of a digital VTR having a frame memory for storing the video signal for one frame will be explained in detail with reference to FIG. 5 and FIGS. 6A to 6E. The example shown here is designed such that the video signal is recorded on a magnetic tape, with each field being divided on two tracks. Accordingly, each track records the video signal for a half field length. This recording scheme is called "two segment recording", in which each field has a recording period which is equal to the time length when the magnetic head drum turns twice.
FIG. 5 shows the scanning trace of the playback head during the quarter tape speed (relative to the normal playback tape speed) slow-motion playback operation superimposed on the track pattern on the magnetic tape 16 used by the digital VTR.
In the figure, a frame of video signal in attention is called "present frame" and given a subscript of 0, the frame which precedes the frame 0 is called "front frame" and given a subscript of -1, and the frame which follows the frame 0 is called "rear frame" and given a subscript of 1. One frame consists of two fields, with the leading field being field 0 and latter field being field 1.
Tracks T.sub.-1 (0.1) to T.sub.-1 (1.2) have a record of video signal of the front frame (frame -1), with the track T.sub.-1 (0.1) recording the former half of field 0 of that frame, track T.sub.-1 (0.2) recording the latter half of field 0, track T.sub.-1 (1.1) recording the former half of field 1 of that frame, and track T.sub.-1 (1.2) recording the latter half of field ,0. Similarly, four tracks T.sub.0 (0.1) to T.sub.0 (1.2) record the video signal of the present frame, with the former two tracks T.sub.-1 (1.2) and T.sub.0 (0.2) recording the field 0 and the latter two tracks T.sub.0 (1.1) and T.sub.0 (1.2) recording the field 1.
The slow-motion playback operation has a different tape speed from that of the normal playback operation, causing the scanning trace of the playback head to be skew with respect to the track, and therefore adjacent two tracks can possibly be overlaid. FIG. 5 show scanning traces S.sub.1, S.sub.3 and S.sub.5 to S.sub.9 of the recorded tracks T.sub.0 (0.1) to T.sub.0 (1.2) for the video signal of the present frame 0 during the quarter-speed slow-motion playback operation. The scanning trace S.sub.1 is produced in the first revolution of the magnetic head drum for the present frame 0, and similarly the scanning traces S.sub.3, S.sub.5, S.sub.7 and so on are those of the third, fifth, seventh, and successive scannings, with the even numbered scanning traces being omitted.
On assumption that the magnetic tape 16 is running at the speed which is 1/4 of the normal playback speed in the direction indicated by the arrow A, the scanning trace S.sub.1 produced by the first drum revolution overlaps with the track T.sub.0 (0.1) in its former half section and overlies the track T.sub.-1 (1.2) in its latter half section during the playback of the video signal for the present frame 0. As the magnetic tape 16 runs further in the direction indicated by the arrow A and the drum rotates for the second turn, third turn and so on, the scanning traces move to the right on the drawing on the magnetic tape 16 as shown by S.sub.5, S.sub.7, S.sub.9 and so on. Consequently, in the first turn of the drum, the scanning trace S.sub.1 overlaps with the hatched area of the last track T.sub.-1 (1.2) of the front frame (frame -1). After that, in successive turns, the overlapping portions of the scanning traces on the track T.sub.-1 (1.2 decrease progressively, and in the fourth turn, the track T.sub. -1 (1.2) is no more scanned, and other tracks T.sub.0 (0.1) and T.sub.0 (0.2) are now scanned in place of the T.sub.-1 (1.2). In the eighth and following turns, the track T.sub.1 (0.1) on which the former half of the leading field 0 of the rear frame (frame 1) is recorded is scanned for playback.
For the scanning trace S.sub.1, the video signal is reproduced on the track T.sub.0 (0.1) in the former half of S.sub.1 and on the track T.sub.-1 (1.2) in the latter half of S.sub.1. For the successive scanning traces S.sub.2, S.sub.3 and so on, the period of video signal reproduction on the track T.sub.-1 (1.2) decreases progressively, and for the scanning trace S.sub.4, the video signal is reproduced on the track T.sub.0 (0 2), instead. The scanning traces S.sub.1 to S.sub.4 cover the entirety of the track T.sub.0 (0.1) for playback, and the scanning traces S.sub.5 to S.sub.8 cover the entirety of the track T.sub.0 (0.2) for playback. The scanning trace S.sub.8 is on the track T.sub.0 (0.1) for the playback scanning. In this manner, the video signal for one field is reproduced through the eight playback scanning operations in the quarter-speed slow-motion playback mode.
Next, storing of the reproduced video signal into the frame memory in the slow-motion playback operation shown in FIG. 5 will be explained with reference to FIGS. 6A to 6E.
FIGS. 6A to 6E show the contents of the frame memory after the head drum has revolved an odd number of turns, with a symbol being appended to tracks of FIG. 5 in correspondence to the contents of record with the intention of indicating the tracks of FIG. 5 where the record is reproduced.
The frame memory has storage areas assigned to the fields (field 0 and field 1), and field records are stored in the respective storage areas in the order of reproduction. The storage areas in the frame memory for the fields (field 0 and field 1) will be called field 0 storage area and field 1 storage area.
For the scanning trace (not shown) which precedes the scanning trace S.sub.1 in FIG. 5 by one, the frame memory stores the reproduced video signal from the track T.sub.-1 (0.1) in the former half of the field 0 storage area and the video signal from the track T.sub.-1 (0.2) in the latter half of the field 0 storage area. The frame memory further stores the reproduced video signal from the track T.sub.-1 (1.1) in the former half of the field 1 storage area and the video signal from the track T.sub.-1 (1.2) in the latter half of the field 1 storage area. With the frame memory storing these records, when the first revolution of the head drum starts for the present frame 0 on the scanning trace S.sub.1, the signal reproduction takes place in about the former half section of the track T.sub.0 (0.1), and the reproduced video signal is stored in about a quarter area following the starting address of the field 0 storage area of the frame memory. In about the latter half section of the scanning trace S.sub.1, the signal reproduction takes place in about the latter half section of the track T.sub.-1 (1.2) (hatched section), and the reproduced video signal is stored in about a last quarter area of the field 1 storage area of the frame memory in FIG. 6A. Since the same video signal as has been stored is recorded, there is no change in the stored contents.
Next, in FIG. 5, when the playback scanning has taken place along the scanning trace S.sub.2 (not shown) and S.sub.3 in the second revolution and third revolution of the head drum, the track T.sub.0 (0.1) has its playback area increasing progressively. Also in the frame memory, in the former half of the field 0 storage area, the track T.sub.0 (0.1) has its storage area for reproduced video signal increasing progressively as shown in FIG. 6B. Simultaneously, the track T.sub.-1 (0.1) of the front frame (frame -1) has its playback area decreasing, and the range of storing in the field 1 storage area of the frame memory decreases progressively.
In this manner, as the magnetic tape 16 runs, the track T.sub.0 (0.2) is rendered the playback scanning. In the fifth revolution of the head drum, the former half of the field 0 storage area of the frame memory stores the video signal which has been reproduced from the entirety of the track T.sub.0 (0.1), and the latter half stores in its about a quarter area the video signal which has been reproduced from the former half of the track T.sub.0 (0.2), as shown in FIG. 6C. In the seventh revolution of the head drum, the track T.sub.0 (0.2) has its playback area expanding, causing the storage area for the video signal from the track T.sub.0 (0.2) in the field 0 storage area of the frame memory to expand as shown in FIG. 6D, and in the eighth and ninth revolutions of the head drum, the field 0 storage area of the frame memory stores the reproduced video signal from the entirety of the track T.sub.0 (0.1), and the field 1 storage area stores the video signal of the field 1 reproduced on the track T.sub.0 (1.1) as shown in FIG. 6E.
The frame memory has its store address for the reproduced video signal specified on the basis of the ID signal which is appended to each block. During the write operation of the frame memory described above, it is read out concurrently. The frame memory is read out alternately for the field 0 storage area and field 1 storage area, and a slow-motion playback video signal of one frame made up of the field 1 and field 0 is retrieved.
The time length in which the head drum turns twice is equal to one field period of the video signal, as mentioned above, and accordingly a four revolution period of the head drum is equal to one frame period. On this account, in the case of the quarter-speed slow-motion playback for the frame memory which has the write operation as shown in FIGS. 6A to 6E, the time length expended for rewriting the playback video signal for one frame is equal to the time length when the head drum makes 16 turns, and accordingly the frame memory is read out four times (16 divided by 4) to produce the video signal for four frames. Namely, during the period when the video signal for one frame is retrieved from the magnetic tape 14, the video signal for four frames is read out of the frame memory, thereby accomplishing the quarter-speed slow-motion playback operation.